Among the methods for laying a telecommunications cable, one of them, the one most generally used, consists firstly in laying the tube inside the previously-dug trench before covering this trench over, the cable then being hauled or pushed inside this tube, either by pulling using an attachment fixed to the end of the cable and the opposite end of which is wound onto a winch or the like beyond the tube, or by blowing compressed air exerting a thrust on a pig secured to the cable and the outside diameter of which is substantially equal to that of the tube in order to obtain relative sealing. An alternative may consist in using a magnetic head for hauling the end of the cable through the tube as a machine which lays the tube in the trench travels along.
However, with these hauling and blowing solutions, the lengths of cable which can be put inside the tube are relatively limited, which, especially in the case where a hauling rope is being used, requires the provision, from point to point, of intermediate chambers making it possible to haul the cable through the tube section by section between two successive chambers and also to boost the hauling force using an intermediate tracked driver, from one section to the next. Furthermore, when the path is curved, the laying of the cable is rendered yet more difficult owing above all to the increase in the hauling or thrusting forces required.
Methods are moreover known in which the cable is laid directly in the ground by means of a machine which simultaneously opens up the ground in order to form a narrow trench into which the cable is unwound from a drum as the machine travels along, the trench then being filled in again behind the machine. In this case, the cable must itself include means which protect it from the environment and must moreover be mechanically reinforced in order to withstand the strains inherent to this laying method, which substantially increases its cost price. Furthermore, once the cable has been placed in its trench and the trench has been filled in again, it is not possible to change the cable itself, except by reopening the trench, which may pose tricky problems, particularly in towns or in a difficult environment.
A third solution, which to some extent combines the previous ones, consists in introducing the cable into its outer protective tube at the factory, for example by extruding the tube continuously around the cable, and in delivering the assembly, wound onto a drum or the like, to the site where it is to be laid in the trench.
This technique makes it possible to retain the advantages of the methods quoted at the beginning, by protecting the cable in the ground and above all allowing the tube which is permanently buried to be reused, it being possible for the cable housed therein optionally to be withdrawn and replaced by another. This same technique makes it possible to avoid the drawbacks of the second methods, the cable not necessarily needing to be reinforced from a mechanical point of view by an outer jacket, generally made of steel, which then requires the conductors of this cable to be protected against the induced currents, which requires earthing, which is a nuisance.
In particular, this solution makes it possible to envisage more favourable economic conditions, avoiding the operations necessary for putting the cable inside the tube on the laying site itself.
However, this last method still exhibits drawbacks, essentially connected with the topography of the environment in which the cable is to be laid, particularly owing to obstacles which may be encountered on the path of the trench accommodating the tube containing the cable, such as crossing carriageways, water courses, railway tracks, etc.. These obstacles in particular require a tunnel or continuous passage to be made in the ground beforehand for the introduction of the tube, preventing the production of open trenches which may, moreover, be prevented for technical reasons or under certain regulations.
Now, the probability of encountering such obstacles is in no way negligible, especially with the current solutions in which the lengths of tube containing the cable to be laid in a single piece now range from 300 m approximately for cables with metal conductors to 2400 m as a general rule for fibre-optic cables.
Owing to the presence of the cable in the tube, the latter cannot be cut at the location of the isolated point encountered. Indeed, on the contrary, in line with this point it is necessary to pay out the tube with the cable inside it in order to gain access to the end on the barrel of the drum, depositing it on the ground especially in a figure eight, in order to avoid phenomena of torsion, before negotiating the obstacle.
The tube with the cable inside thus paid out may be laid through the obstacle by driving or some other method, making continuous passage through which the total length of cable has to be passed. The tube with its cable is then repackaged on the drum in order to resume the operation which consists in burying it at the bottom of the trench. The object of this operation is to avoid splicing the cable, necessary in the case of the tube being cut; in actual fact, the cable cannot be pulled out of the tube owing to the friction of one on the other on the drum or on the ground when the tube is arranged on the latter in figure eight. Additional splicing is moreover always detrimental to the technical and economical balance of the installation for laying such a cable.
It is therefore understood that these operations require significant manpower, numerous interventions on the cable and on the tube and sufficient space for laying out on the ground. This results in risks to the cable itself, especially when it consists of optical fibres, and in all cases involves a significant cost and above all a detrimental cutting of the cable. Moreover, in practice, this solution can be envisaged only in limited cases, for lengths of tube having to negotiate just one obstacle and when the latter is only a few hundreds of metres from one of the ends of the length to be laid.